Multi-stage combustors are used particularly in lean burn fuel systems of gas turbine engines to reduce unwanted emissions while maintaining thermal efficiency and flame stability. For example, duplex fuel injectors have pilot and mains fuel manifolds feeding pilot and mains discharge orifices of the injectors. At low power conditions only the pilot stage is activated, while at higher power conditions both pilot and mains stages are activated. The fuel for the manifolds typically derives from a pumped and metered supply. A splitter valve can then be provided to selectively split the metered supply between the manifolds as required for a given staging.
A typical annular combustor has a circumferential arrangement of fuel injectors, each associated with respective pilot and mains feeds extending from the circumferentially extending pilot and mains manifolds. Each injector generally has a nozzle forming the discharge orifices which discharge fuel into the combustion chamber of the combustor, a feed arm for the transport of fuel to the nozzle, and a head at the outside of the combustor at which the pilot and mains feeds enter the feed arm. Within the injectors, a check valve, known as a flow scheduling valve (FSV), is typically associated with each feed in order to retain a primed manifold when de-staged and at shut-down. The FSVs also prevent fuel flow into the injector nozzle when the supply pressure is less than the cracking pressure (i.e. less than a given difference between manifold pressure and combustor gas pressure).
Multi-stage combustors may have further stages and/or manifolds. For example, the pilot manifold may be split into two manifolds for lean blow-out prevention during rapid engine decelerations.
During pilot-only operation, the splitter valve directs fuel for burning flows only through the pilot fuel circuit (i.e. pilot manifold and feeds). It is therefore conventional to control temperatures in the stagnant (i.e. mains) fuel circuit to prevent coking due to heat pick up from the hot engine casing. One known approach, for example, is to provide a separate recirculation manifold which is used to keep the fuel in the mains manifold cool when it is deselected. It does this by keeping the fuel in the mains manifold moving, although a cooling flow also has to be maintained in the recirculation manifold during mains operation to avoid coking.
FIG. 1 shows schematically a combustion staging system 130 for a gas turbine engine. A metered fuel flow arrives at the staging system at a pressure Pfmu. The staging system splits the fuel into two flows: one at a pressure Pp for first 131a and second 131b segments of a pilot manifold and the other at a pressure Pm for a mains manifold 132. Fuel injectors 133 of a combustor of the engine are split into two groups. The injectors of one group are connected to the first pilot manifold segment 131a, while the injectors of the other group are connected to the second pilot manifold segment 131b. The mains manifold feeds secondary nozzles of the fuel injectors. Pilot FSVs 139 and mains FSVs 140 at the injectors prevent combustion chamber gases entering the respective manifolds. By varying the fuel split between the manifolds, staging control of the engine can be performed.
In more detail, the staging system 130 has a fuel flow splitting valve (FFSV) 134, which receives the metered fuel flow from the HMU at pressure Pfmu. A spool is slidable within the FFSV under the control of a servo-valve 135, the position of the spool determining the outgoing flow split between a pilot connection pipe 136 which delivers fuel to the pilot manifold segments 131a, b and a mains connection pipe 137 which delivers fuel to the mains manifold 132. The spool can be positioned so that the mains stage is deselected, with the entire metered flow going to the pilot stage. An LVDT 138 provides feedback on the position of the spool to an engine electronic controller (EEC), which in turn controls staging by control of the servo-valve.
Between the FFSV 134 and the second pilot manifold segment 131b, the pilot connection pipe 136 communicates with a lean blow out protection valve 150 which controls communication between the pilot connection pipe 136 and the second pilot manifold segment 131b. The lean blow out protection valve is spring biased towards an open position. A solenoid operated control valve 152 is operable to apply a control pressure to the valve member of the lean blow out protection valve to move it against the action of the spring biasing to a closed position, interrupting the communication between the pilot connection pipe 136 and the second pilot manifold segment 131b, when required. Accordingly, if there is only a pilot delivery of fuel to the engine and there is a concern that a lean blow out condition may occur, the lean blow out protection valve 150 can be closed by appropriate control of the solenoid operated control valve 152, with the result that fuel delivery to the second pilot manifold segment 131b is restricted, whilst that to the first pilot manifold segment 131a is increased. Adequate pilot delivery can therefore be assured (albeit through a reduced number of the injectors 133), resulting in a reduced risk of a lean blow-out condition occurring.
The staging system 130 also has a recirculation line to provide the mains manifold 132 with a cooling flow of fuel when the mains manifold is deselected. The recirculation line has a delivery section including a delivery pipe 141 which receives the cooling flow from a fuel recirculating control valve (FRCV) 142, and a recirculation manifold 143 into which the delivery pipe feeds the cooling flow. The recirculation manifold has feeds which introduce the cooling flow from the recirculation manifold to the mains manifold via connections to the feeds from the mains manifold to the mains FSVs 140.
In addition, the recirculation line has a return section which collects the returning cooling flow from the mains manifold 132. The return section is formed by a portion of the mains connection pipe 137 and a branch pipe 144 from the mains connection pipe, the branch pipe extending to a recirculating flow return valve (RFRV) 145 from whence the cooling flow exits the recirculation line.
The cooling flow for the recirculation line is obtained from the HMU at a pressure HPf via a cooling flow orifice (CFO) 146. On leaving the RFRV 145 via a pressure raising orifice (PRO) 147, the cooling flow is returned to the pumping unit for re-pressurisation by the HP pumping stage. A check valve 148 accommodates expansion of fuel trapped in the pilot and mains system during shutdown when the fuel expands due to combustor casing heat soak back. The check valve can be set to a pressure which prevents fuel boiling in the manifolds. The FRCV 142 and the RFRV 145 are operated under the control of the EEC. The HMU also supplies fuel at pressure HPf for operation of the servo-valve 135, the RFRV 145, and the lean blow out protection valve 150.
When the mains is staged in, a cooling flow is also directed through the recirculation manifold 143 to avoid coking therein. More particularly a small bypass flow is extracted from the HMU's metered fuel flow at pressure Pfmu. The bypass flow is sent via a flow washed filter 149 to a separate inlet of the FRCV 142, and thence through the delivery pipe 141 to the recirculation manifold 143. The bypass flow exits the recirculation manifold to rejoin the mains fuel flow at the injectors 133.
In such a system, the fuel pressure in the mains manifold needs to be maintained above the combustion chamber gas pressure (P30) to prevent P30 gas ingestion into the fuel system, which is a potentially hazardous failure mode. In pilot-only mode, it also should be maintained below the cracking pressure of the mains FSVs 140 (i.e. below a given difference between manifold pressure and P30) to prevent unmetered flow into the combustion chamber via the mains injectors, which is a fuel coking risk. However, it can be difficult to meet these pressure requirements across the operating envelope of the system due to leakage through the FFSV 134 from Pfmu, and variation in the tolerances of the CFO 146 and the PRO 147.
A further complication can be system pressure ripple which acts to reduce the available margins as it is proportional to the manifold pressure, which increases with total fuel flow.